Near-island enhancement in mesopelagic micronekton assemblages off Hawaiʻi

The west coast of Hawaiʻi Island hosts elevated primary production compared to offshore waters and an abundance of large pelagic animals for reasons that are not entirely understood. Here we show that the nearshore environment off the west coast of Hawaiʻi exhibits an increased biomass and abundance of mesopelagic micronekton. Acoustic surveys from this study and prior work show a higher nautical area scattering coefficient (as a proxy for biomass) in a deep non-migratory layer in nearshore sites compared to offshore sites that is persistent over five years. Cobb trawl samples taken at the depths of the deep scattering layer (∼450 to ∼550 m) in 2016 and 2017 showed 1.3 to 2.2 times higher biomass and 2.7 times higher abundance nearshore (∼4 km from shore) compared to offshore (∼24 km from shore). Fishes dominated the trawl catches and a large fraction of the nearshore enhancements were due to Sternoptychidae and Serrivomeridae across both years. In contrast, Melamphaeidae consistently were more abundant and had greater biomass offshore. This deep scattering assemblage contrasts with the mesopelagic boundary layer assemblage by being nonmigratory and taxonomically different in composition. These nearshore mesopelagic enhancements could occur as the result of increased nearshore food supplies deriving from Island Mass Effect enhanced primary production or from advection and concentration in a complex flow environment in the lee of the island. Regardless of mechanism, this temporally persistent, high biomass, largely nonmigratory layer of mesopelagic micronekton is different from the better known migratory mesopelagic boundary layer assemblage and may provide food to deeper diving marine mammals and pelagic fishes possibly explaining the aggregation of large pelagic animals in this region.